A number of diseases can severely weaken the immune system, notably certain cancers, diabetes, and HIV infection. So can certain drugs, such as corticosteroids (for inflammation) and treatments for cancer, hepatitis, and organ transplant recipients. Anyone with severely weakened immunity is especially susceptible to potentially deadly infections and should take the following special precautions:

Precautions for vulnerable people

Get vaccinated against the flu and pneumonia. But don't get vaccinated against chicken pox, smallpox, measles, mumps, or rubella. Unlike the flu and pneumonia vaccines, those vaccines contain live viruses that can overwhelm weakened defenses.

Practice scrupulous dental hygiene to ward off periodontal infections. Cook meat, fish, and eggs thoroughly to kill the bacteria that can lurk in food. Consider installing a water filter to extract parasites from your water.

Apply insect repellent assiduously when outdoors, especially in the woods, to ward off infection with West Nile and Lyme diseases. Be especially alert to the appearance of any new symptoms, including common ones like back pain or a rash, since that could signal an underlying and potentially dangerous infection.

Minimize hospital visits, since germs are especially plentiful and virulent there. If you're hospitalized, insist that hospital staff follow the proper precautions to prevent the spread of infection. Consider donating your own blood before any surgery that may require a blood transfusion, to eliminate the possibility of a blood-borne infection.

Ask whether you should take antibiotics before undergoing periodontal work and certain major operations, notably hysterectomy and cardiac, colorectal, or joint replacement surgery.